SBP expansion chamber header pipe length for best performance

[Frickard]

Hey guys,

I ordered the expansion chamber from SBP and I've been doing a bit of reading on the topic of extending the pipe before the expansion chamber for optimal performance. I've got a Grubee Starfire 66cc engine on the way as well. I'll be putting an aftermarket intake filter as well as a billet intake on the engine. Anyone know how much pipe there should be before the expansion chamber? Or is this relative to each individual bike? Sorry, I don't know much about exhaust mechanics :x I'm pretty much learning as I go. Any help is greatly appreciated.

Cheers

 

[Drewd]

its really individual to the motor but if our motors were all standardized with consistent quality then you'd be able to say that a certain distance was a good starting point.

i bought extra copper pipe from a hardware store and trimmed it into several lengths and swapped them until i found a length that i liked. mine are tuned for mid range rpm. my engines hit a certain rpm and won't give any more. this gives me somewhat more low end torque which i value for hill climbing and the extra power in the midrange allows me to use gearing for my top speed. i had it shorter once for more power at higher revs but it killed my low end torque. i didn't want to keep my engine at these higher revs just to enjoy the motor so i compromised. if i know i will be doing a lot of hill climbing at low speeds, i'll just swap out the 3/4 inch copper pipe section for a longer piece of pipe for more low end torque.

 

[F_Rod81]

I have a setup similar to your description and like drewd had mentioned. It depends on what you want in performance.

Longer = low-end & shorter = top-end

You need to find a good medium for an all-around power band

 

[DaveC]

Because of a screwed up frame design I couldn't use the supplied pipe from SBP. There's not enough room between the lower frame and front tire, to the point of hitting my foot when I turn corners if I'm not careful. I used a copper flex pipe made for hooking up a hot water heater. It bends fairly easily. I have it bending from the header down and to the right to meet up with the expansion chamber. Its about 12" long.

 

[Venice Motor Bikes]

What F Rod said about shorter = top end & long = low end power is true.
The stock length of the pipe is great as it is for lower RPM power, & I've cut the header pipe down to 6" & it worked great for screaming hihg RPM power.
Now keep in mind that if you want screaming top end?, you'll have to do many other mods to the engine!

 

[MotorNut123]

so if you cut it in the middle length, then ull get even power through the whole range?

 

[Alson333]

I remember when I had my SBP expansion chamber, they had in the instructions that theres a header pipe that was 21/2 inches long, thats the one you shorten or leave alone, I took a 1/2 inch off and it seemed for mid range, I left it there. My friend realy tested alot of sizes and he went back to two inches which is a 1/2 inch off the origenal size

 

[Drewd]

so if you cut it in the middle length, then ull get even power through the whole range?

Try full length and then trim 1/3 inch at a time to get the performance you desire. I bought a small section of 3/4 inch copper pipe form a box store and cut it into various sizes to test. On my older build, I have two pieces of copper pipe, a shorter one for high rpms (great for cruising on flat trails or city streets and a longer one for hill climbing in the mountains.

Ensure engine is fully warmed up for accurate test of how header length is affecting your engine.

It is not unusual to need to use a larger jet with a tuned pipe but that also depends where you live. Stock jetting (#70) on NT carb is fairly rich at my altitude (6400-6600 feet range) but a tuned pipe leans it out perfectly.

One way to check jetting is to do following: Find a nice steep hill, warm engine up, accelerate to full throttle and shut fuel line off-using a steep hill limits your top speed so test can be done easily. If you are too lean, engine will just die abruptly when it runs out of fuel, if you are too rich, engine will speed up abruptly before it dies, if mixture is just right, it may speed up a little (not much) or die gradually (not as abruptly if it was too lean).

 

[bairdco]

from what i've read and experienced, 10" gives you a good powerband, with the pipe coming on strong on the top end.

i'm in westminster, by the way. if you want it welded up instead of using copper pipe, silicon tubing, or any other jerry rigged set-up, lemme know.

my fee is a 12 pack...

 

[Alson333]

I see BikeBerry has a new expansion chamber fo 65 bucks, its made to go on the right of the bike upward, it quite its go a longer silencer, the video said its good for lor to mid range, ya ought to check it out

 

[maintenancenazi]

from what i've read and experienced, 10" gives you a good powerband, with the pipe coming on strong on the top end.

i'm in westminster, by the way. if you want it welded up instead of using copper pipe, silicon tubing, or any other jerry rigged set-up, lemme know.

my fee is a 12 pack...

Hey Baird, thanks for sharing your insight! Your experience with HTs is gold. Question, that 10'' you cite, is that measured starting from the cylinder head port, or from the end of the SBP flange that bolts to the head?

Peace, James

 

[bairdco]

measured from the flange.

10" is a good starting point based on the calculations. a little more, a little less, probably won't matter much unless you have a dyno and you're tuning it for an exact powerband.

once you've got the length you want, you need to be carefull with your mixture. too lean and you'll melt your piston.

with an xchamber, the hot gasses don't get expelled out the pipe, and your engine runs a lot hotter.

more oil, a colder plug, and a bigger head is the way to go.

 

[KCvale]

I have done a few SickBikes X-chamber high performance builds and there are a lot of factors.

First is motor size, then how you can route the pipe.
Here are a couple of examples of different ways I have used.

In short the straight piece of copper tube is tuned to 48cc, cut it in half for a 66cc motor and disregard the re-jetting.
I still scratch my head with that advice as I have never had to re-jet a carb ever, maybe readjust the needle length but that's it and yet still achieve outstanding performance with just good fuel, an Iridium plug, and the X-pipe.

Just my experience, take it or leave it but regardless an X-chamber will be the most performance enhancing item you can buy for there motors even if it's not exactly turned.

And since I know someone is going to ask about that second pic of Bad Mojo ya, I got real custom on that with the car muffler custom built from my local muffler shop on the end.
Long story about that but that bike kicked ass with a 3-speed and sounded more like a Harley than a chain saw ;-}

 

[maintenancenazi]

measured from the flange.

10" is a good starting point based on the calculations. a little more, a little less, probably won't matter much unless you have a dyno and you're tuning it for an exact powerband.

once you've got the length you want, you need to be carefull with your mixture. too lean and you'll melt your piston.

with an xchamber, the hot gasses don't get expelled out the pipe, and your engine runs a lot hotter.

more oil, a colder plug, and a bigger head is the way to go.

^^ Thanks for the info Baird! Yea my current header pipe is about 13'' total, from end of flange to the inlet of the chamber. Seems to have very good all around power at this length. I would like to try and shorten if a little more, but in my install, my header pipe wraps around the front downtube, and then back ( pipe is mounted in the "up" position ). It's so close to the downtube you can just barely slip a cigar paper between the head pipe and downtube! So unless I figure another position, I guess I am stuck where I'm at. Oh yes, already have my Puch hi-hi head installed, plus at my weight ( 230 lbs! ) and gearing 32 tooth rear on a 26'' wheel, that loads the engine quite a bit. So I have ended up going all the way to a B10HS NGK, as well as adding a little octane booster to prevent ping. Jetting seems spot on now, just barely four strokes under light load. And done a few piston, and porting mods as well.

Although it's not "official" friends in cars, and other moped riders have told me that I top out at about 45 mph! I know one thing, going that fast on a bicycle feels like going 80 on my motorcycle! I normally don't go this fast however, and am just content to cruse around 25-30, nice fun speed...

Peace, James

And nice pics KC! Those all look great, thanks for sharing....

 

[maintenancenazi]

Here's a pic, before my new head install however...

 

[maurtis]

Here's a pic, before my new head install however...

That chain looks a little on the loose side, but I like how you wrapped the exhaust around. How hot does it get? Fried juevos con cursing?

 

[maintenancenazi]

That chain looks a little on the loose side, but I like how you wrapped the exhaust around. How hot does it get? Fried juevos con cursing?

Good eye there, when that pic was taken it was waaaay too loose, but managed to take my half link out and now it's just right! And I know that pipe looks real sketchy where it's at, but on the install I got it tucked in real close to the frame. plus being narrow in design, it really doesn't interfere with peddling at all. Plus when you shut down the engine it seems to cool down fairly quick. I have been running this pipe that way since February, and ridden it most every day, and it has not burned me yet!

Peace, James

 

[KCvale]

Pipe against the frame is a good thing, it's a big heat sink ;-}

 

[mountain80]

Just so I am clear, do I understand that 12" from the exhaust flange to where it meets the 2nd part of the pipe is a good starting point, want to make sure as I am building a stainless exhaust flange that has been die formed to transition from the square port to a round pipe for coupling as well as a stainless header and I would have to account for the extra length that goes inside the second part(expansion chamber) yes I will shorten it afterwards to tune for rpm range desired namely solid midrange. Just need to know exactly how this measurement is realized.
Cheers.